The present invention relates to an ultrasonic probe used for ultrasonic diagnostic apparatus.
Ultrasonic diagnostic apparatus are known which radiate ultrasonic pulse beams into an object to be examined, receive the echoes which are reflected by the boundary of the structures of organisms in accordance with a difference in acoustic impedance, displaying them on a display such as a cathode-ray tube, and observing the structure of the organisms of the object to be examined from the displayed image of a desired tomogram. Since they enable the interior of the body to be diagnosed from the exterior, they are widely used.
An ultrasonic probe having an electroacoustic conversion element is used in order to transmit and receive ultrasonic waves. An ultrasonic exciting signal of a desired frequency is supplied to the electroacoustic conversion element, whereby ultrasonic beams are radiated from the exited element and the reflected echo is converted into an electrical signal.
Lead zirconate titanate (PZT) piezoelectric ceramic plates are conventionally widely used for the electroacoustic conversion portions. Lead zirconate titanate is a ferroelectric substance which has a spontaneous polarization, and exhibits strong piezoelectricity when the polarities are aligned in perpendicular to the ceramic plate by applying a strong electric field perpendicularly to the ceramic plate surface at a predetermined temperature, namely, by poling. Since a PZT ceramic exhibits ferroelectricity up to the temperature limit which is sufficiently higher than room temperature and has strong coercive field, remanent polarization after poling is retained stably. Therefore, even if the ambient temperature changes or electric field is applied, the piezoelectricity of a PZT ceramic hardly changes unless there is a great change in the ambient temperature or the electric field. A conventional ultrasonic probe obtains stable electroacoustic conversion efficiency utilizing this characteristic.
With a recent demand for a higher-capacity ultrasonic diagnostic apparatus, however, it is sometimes desirable to externally control the conversion characteristic of the electroacoustic conversion portion of an ultrasonic probe. For example, in a conventional electronically scanning linear array ultrasonic probe shown in FIG. 1, the ultrasonic beam pattern viewed in the direction of array (in the longitudinal direction L) of each of the strip-shaped transducer elements 12 held by a backing member 11 is controlled by varying the number of transducer elements to be driven (in this case, the aperture of the linear array probe) and the phase of an electrical signal to be applied.
However, since the aperture and, hence, the position of the focal point is fixed with respect to the direction (transverse direction S) perpendicular to the direction of array (longitudinal direction L) of the transducer elements, it is impossible to control the resolution in the transverse direction S. Therefore, it is sometimes impossible to obtain sufficient resolution according to particular positions (probing depths) of objects to be examined. This problem is solved by weighting the distribution of the conversion efficiency by externally controlling the electroacoustic conversion efficiency in each transducer element, but this solution is difficult with respect to the above-described transducer element made of a PZT piezoelectric ceramic.
A method for solving this problem is disclosed in Japanese Patent Laid-Open No. 21057/1981. In this method, each of the electrodes on the upper and lower surfaces of the transducer elements are divided in the longitudinal and transverse directions, so that the aperture in the transverse direction of the probe is also variable. The effects brought about by this method, however, may be insufficient, because, even if a driving electric field is selectively applied to each divided electrode on the upper and lower surfaces of electroacoustic conversion elements, the vicinities thereof may also be excited by a leakage electric field.